It is well known that motor, directly driving robot arm rotational joint without reduction gear assembly, allows very precise position control. However, the motor size must be large in order to provide sufficient torque and high resolution motor shaft encoder is required for use in end point accuracy. Although small motor with speed reduction gears is able to give higher torque, high repeatability is hard to achieve due to backlash existing between the gears of speed reduction gear assembly. Encoder of high resolution can be applied at the end point to increase accuracy but the feedback position control of high bandwidth will be difficult to achieve if there exists backlash which leads to vibration of motor shaft all the time. Thus, if there is a way to use small motor with backlash eliminated, speed reduction gears can be introduced to increase torque with the presence of feedback position control of high bandwidth.
On the other hand, right angle transmission for robot arm joint facilitates the design of robot arm of compact size because the motor is able to be put along the robot arm to provide torque to the joint through right angle transmission.